Hardening SUJ2 Bearing Steel (100Cr6 / AISI 52100) for Small Parts

SUJ2 (JIS G 4805) is a through-hardening bearing steel with high carbon (~0.95–1.10%) and chromium (~1.30–1.60%). It is equivalent to 100Cr6 (EN), AISI 52100 (SAE) and GCr15 (China).

The high carbon and chromium give a fine dispersion of carbides and let the steel harden all the way through to ~60–62 HRC, with good wear resistance, contact hardness and dimensional stability — which is why it is used for pins, rollers, balls, bushings and small shafts.

The international reference is ISO 683-17, which covers the through-hardening group of ball and roller bearing steels.

Austenitizing: heat to about 830–860°C and soak so the right amount of carbide dissolves (not so much that grains coarsen). Furnace atmosphere must be controlled to prevent surface decarburization.

Quench: oil-quench (or another suitable medium) to form martensite. As-quenched hardness is high, around 63–65 HRC, but with retained austenite and high brittleness.

Temper: a low temper at about 150–180°C reduces brittleness while holding ~60–62 HRC. A higher temper trades hardness for more toughness where needed.

High-carbon steels like SUJ2 usually keep some retained austenite after quenching. It is softer than martensite and unstable — over time, or under service temperature and load, it slowly transforms to martensite and the part grows slightly. That is a serious problem for bearings, gauges and tight-tolerance parts.

When to sub-zero treat: for parts needing high dimensional stability or maximum hardness, deep-freeze at about −70 to −80°C (or lower, cryogenic) immediately after quenching to convert more retained austenite to martensite, gaining a little hardness and a more stable size.

Always temper after sub-zero, because the freshly formed martensite is brittle; tempering restores toughness and relieves stress.

SUJ2 is common in small parts that see local contact and wear — dowels/pins, rollers, bushings, small shafts and balls. Through-hardening gives uniform hardness across the section, right for contact surfaces carrying Hertzian loads.

The challenge in high-count lots is hardness consistency across every piece, distortion of thin parts and preventing decarburization during heating. A controlled-atmosphere mesh-belt furnace heats large volumes of small parts evenly and continuously.

Tight-tolerance parts are usually ground after hardening, so allow grind stock and state the final tolerance clearly. (V.S. Heat Treatment handles small parts only — not large items such as gears or big shafts.)

Specify: grade (SUJ2/100Cr6/52100), target hardness range (e.g. 60–62 HRC), any dimensional-stability requirement (sub-zero or not), distortion limits, grind allowance, and weight/count per lot.

Hardness can be reported as HRC, or measured/converted as HV on a prepared cross-section, random-sampled per lot with a certificate. For very thin parts where an HRC indenter punches through, specify HV.

Do not set an unnecessarily narrow hardness band — it raises rejection and cost. If unsure, state the function so the team can set a repeatable criterion.